1. Field of the Invention
In general this invention is related to devices for cutting precision angles in a workpiece; in particular, the invention describes a new and improved miter gauge that combines exact-alignment features allowing a degree of precision within seconds of a degree in woodworking.
2. Description of the Related Art
The ability to cut precise angles to fit adjoining parts is critical to machinists and woodworkers. Picture frames, windows, doors and furniture are common examples of articles of manufacture that are assembled from parts having mitered joints that require a very close fit in order to result in an acceptable product. Thus, several tools have been developed over the years to aid woodworkers in obtaining such precise cuts.
The miter gauge is a tool designed to achieve this goal that is normally provided as standard equipment with table saws. It consists of a guide bar that rides horizontally in parallel to the blade in a machined miter slot on the top of the table saw; a fence assembly that rotates around a pivot axis through the guide bar and perpendicular to the top of the table saw; a protractor scale for setting the fence assembly to various angles with respect to the saw blade; and a lock knob to hold the fence in the desired position. The machined miter slot on the top of the table saw is parallel to the blade, so that the guide bar sliding within it is also kept parallel to the blade at all times. The protractor scale is adapted to utilize the guide bar as a reference point for measuring the angles at which the fence assembly is set with respect to the blade as it is rotated around the pivot point. Some miter gauges also have built-in stops to allow quick settings to the more commonly used angles.
Thus, the performance of a miter gauge depends on how well the guide bar fits in the miter slot in the table saw and on how accurately the fence's angle can be set. Any play between the bar and the slot may result in lateral movement of the bar as it slides up and down the slot, which in turn produces a variation in the angle of the fence with respect to the blade affecting the accuracy of the cut. Similarly, the fence's angle is set by visually matching a position marker on the fence assembly with a mark on the protractor scale and the procedure necessarily involves a subjective determination in finding the correct position. Thus, in practice, these two sources of error have greatly reduced the theoretical accuracy of standard miter gauges and a variety of improved tools and accessories have been developed for specific purposes.
Some gauges use the notion of complementary angles to achieve accuracy in cutting 45-degree angles for adjoining parts. A 90-degree square is mounted on the fence of the conventional miter gauge and set with both faces at approximately 45 degrees with respect to the blade. One workpiece is cut by using one face of the square and the adjoining workpiece by using the other face, thus ensuring a complementary fit between the two pieces (that is, the angles of both cuts necessarily add to 90 degrees) even if each cut is not exactly at 45 degrees. A variety of these tools is shown on page 74 of the April 1992 issue of "Fine Woodworking" magazine, published by The Taunton Press, Inc., of Newtown, Conn.
These accessories are limited in their use because they only address the issue of ensuring a good fit between complementary angles. They do not improve the precision of miter gauges in cutting angles in general, which is still dependent on the precise parallel alignment of the guide bar with the saw blade, the precise alignment of the protractor scale with the guide bar, and the precise setting of the fence at the desired angle as measured by the scale. These three sources of error remain unimproved when the gauge is used for non-complementary cuts.
Another type of improvement is illustrated on page 60 of the August issue of the magazine referenced above. It consists of a sine-bar assembly used as an accessory with a standard miter gauge to set the angle of the fence to a predetermined precise quantity. In order to utilize the accessory, a second miter slot on the top of the table saw is required, also parallel to the blade, and it is used with retaining wedges to align a conventional carpenter square at right angle with the blade. A sine bar and gauge blocks are then used to form known angles with the square and correspondingly to set the angle of the miter-gauge fence. The sine-bar assembly is then removed and the miter gauge is used in a conventional way to cut at the selected angle.
This gauge accessory provides improved precision over the performance of standard miter gauges, but it is cumbersome to use because of the various separate components and steps involved in setting the fence to the desired angle. First, the square has to be aligned with the second miter slot in the table (which requires the use of retaining wedges firmly held in the slot); second, the fence of the miter gauge is to be aligned with the square; finally, the sine-bar assembly is placed between the square and the fence and used to rotate the fence to the desired angle while all other parts are held stationary. This device in theory eliminates the subjective selection of the precise position of the fence by substituting its visual positioning with the introduction of an exact gauge block of predetermined thickness. In fact, though, each step requires a precise alignment of movable components and, therefore, represents a potential source of error.
In summary, all of the improvement devices found in the prior art retain most of the problems of standard miter gauges. Therefore, there is still a need for an improved miter gauge that permits the setting of the fence to precise angles with a minimum of movable parts and steps, which in turn results in minimal alignment errors.